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We don’t usually think of agricultural landscapes as ecosystems, but despite their anthropogenic origin they have much in common with our planet’s more natural ecosystems. And like those natural spaces, agricultural ecosystems are becoming less biodiverse. We’re in the midst of a biodiversity crisis, and it is evident on farms just as in rainforests. Stemming the loss of biodiversity in the Amazon jungle is no easy task, but it turns out that adjusting the plants that comprise our nation’s farmland is a lot more straightforward.

Most scientific biodiversity research has focused on the more pristine of our planet’s surface, despite the fact that quite a large proportion of our planet is devoted to growing crops. In the continental United States, cropland covers 408 million acres (165 million hectares), which is nearly a quarter of our land. In some areas, like the Great Plains and the Corn Belt, more than 50% of land area is devoted to growing the food we eat.

Like fields and forests, diverse agricultural ecosystems (or, agroecosystems) are better able to withstand external stresses. They’re more resilient in the face of the extreme weather brought about by climate change, and they are more likely to offer natural forms of pest control. As agroecosystems become more uniform, the pressure from pests increases along with the need to introduce harmful pesticides into the ecosystem. They’re more likely to withstand disease and drought. More complex agricultural landscapes are also beneficial for wildlife biodiversity. Butterflies and birds, for example, benefit from a heterogeneous landscape, even if primarily anthropogenic in nature.

To find out just how homogeneous America’s farmlands are, Kansas State University researcher Jonathan Aguilar, together with colleagues from North Dakota State University and the US Department of Agriculture, turned to a census administered to farmers every five years by the USDA. They used the entire portion of the dataset that’s been digitized, from 1978 through to 2012.

Crop diversity increased nationwide from 1978 to 1987, but since then it has consistently decreased, and most sharply between 1992 and the present. By 2012, crop diversity was statistically lower than it was 35 years prior. In part, that can be attributed to technological improvements making cotton, corn, and soybean farming more efficient, but market forces are also at place. Ethanol policy in the US and demand for US soybeans in China have resulted in more acres of corn and soy, while farmers have cultivated fewer wheat fields and other crops in response.

While agroecosystem biodiversity is down overall compared to 1978 levels, when Aguilar considered each of the USDA’s nine farming regions individually, it became clear that the trend is driven more by some regions than by others. In three regions, crop diversity was indeed lower in 2012 than in 1978, but in five regions it was statistically indistinguishable, and farmlands surrounding the Mississippi River are actually more diverse now than they were then. Aguilar and his team even discovered differences in biodiversity at a county-by-county level, owing perhaps to differences in local weather patterns or legislation.

Increasing agricultural diversity isn’t just beneficial for farmland ecosystems and the wildlife that lives within it; it’s also smart business. A diverse farm increases the farmer’s chances of avoiding widespread crop failures. For example, a farmer might plant corn on part of her property and sorghum, an important component of animal feed, on another. If the season was unusually dry, the more drought-tolerant sorghum would still grow, even if the corn crop failed. That would allow the farmer to generate more revenue than if she had blanketed the entire farm in corn during such a low-yield season.

While agroecosystems and natural ecosystems have much in common, there is one important difference: farms are typically replanted every year. “This means that, theoretically, crop species diversity can change relatively rapidly, so the potential for swift positive change is considerable, unlike in natural ecosystems,” writes Aguilar. Given the threat posed by climate change, he and his team argue that policy changes that promote crop diversity may be needed to ensure that we remain well fed in the future. – Jason G. Goldman | 18 September 2015